On 2013/02/18 12:37 PM, Adam Ryczkowski wrote:
...
to migrate btrfs from one partition layout to another.
...
<source> sits on top of lvm2 logical volume, which sits on top of
cryptsetup Luks device which subsequentely sits on top of mdadm RAID-6
spanning a partition on each of 4 hard drives ... is a read-only
snaphot which I estimate contain ca. 100GB data.
...
<destination> is btrfs multidevice raid10 filesystem, which is based
on 4 cryptsetup Luks devices, each live as a separate partition on the
same 4 physical hard drives ...
...
about 8MB/sek read (and the same speed of write) from each of all 4
hard drives).
I hope you've solved this already - but if not:
The unnecessarily complex setup aside, a 4-disk RAID6 is going to be
slow - most would have gone for a RAID10 configuration, albeit that it
has less redundancy.
Another real problem here is that you are copying data from these disks
to themselves. This means that for every read and write that all four of
the disks have to do two seeks. This is time-consuming of the order of
7ms per seek depending on the disks you have. The way to avoid these
unnecessary seeks is to first copy the data to a separate unrelated
device and then to copy from that device to your final destination device.
To increase RAID6 write performance (Perhaps irrelevant here) you can
try optimising the stripe_cache_size value. It can use a ton of memory
depending on how large a stripe cache setting you end up with. Search
online for "mdraid stripe_cache_size".
To increase the read performance you can try optimising the md arrays'
readahead. As above, search online for "blockdev setra". This should
hopefully make a noticeable difference.
Good luck.
--
__________
Brendan Hide
http://swiftspirit.co.za/
http://www.webafrica.co.za/?AFF1E97
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